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- Newsgroups: comp.sys.amiga.audio
- Path: sparky!uunet!usc!sol.ctr.columbia.edu!src.honeywell.com!hwcae!ericj
- From: ericj@hwcae.Honeywell.COM (Eric Jacobsen)
- Subject: Re: Halving sample speed while keeping pitch
- In-Reply-To: djs@jet.uk's message of 26 Aug 92 14: 54:36 GMT
- Message-ID: <ERICJ.92Aug26131424@lagos.cfsat.Honeywell.COM>
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- Organization: Honeywell, Air Transport Division; Phoenix, AZ
- References: <9208241809.AA00474@.nairobi.inel.gov.inel.gov.>
- <ERICJ.92Aug24131911@lagos.cfsat.Honeywell.COM>
- <1992Aug26.145436.5222@jet.uk>
- Date: Wed, 26 Aug 1992 19:14:24 GMT
- Lines: 42
-
- In article <1992Aug26.145436.5222@jet.uk> djs@jet.uk (David J Stevenson) writes:
-
- >2. Take the upsampled signal and mix it with a sine wave that shifts
- >the pitch back up. (You get a sum and difference frequency, you filter
- >out the difference freq). Actually, this is kinda tough to do
- >digitally. It is a lot easier to do this in the analog domain when
- >you play it back.
- Eh? What pitch sine wave? At what level?
- What the software actually needs to do is double the number of samples
- in the signal; this is what oversampling does. So, between every sample
- in the source signal add a sample at the zero level (may be zero, or 32768
- or whatever, depending on number of bits and whether the signal data is
- signed). This gives the correct number of samples, but has introduced some
- high-frequency distortion.
-
- Ayup. Adds a lot of unnecessary crap to an otherwise clean signal. A real
- upsampler (or oversampler as you call it) interpolates the missing samples
- so you have the same nice clean signal you started with, only with more
- samples (but no added bandwidth). Even so, doing this doesn't provide the
- time stretch with pitch preservation that the original poster was looking
- for.
-
- However, this distortion is ABOVE the maximum frequency present in the
- original signal (see Nyquist theorem). It can therefore be removed by a
- simple filter. How to design digital filters is a complex subject to
- understand, but the algorithms for designing filters are available in the
- public domain. Search the comp.dsp FAQ for references on filter design.
-
- DSP design is a lot fun, though. It's what I do for a living.
-
- --
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